The present invention relates to a device in an optical head apparatus for recording information on an optical disk as a recording medium and reproducing information from the optical disk.
An optical disk apparatus includes an optical head apparatus having an objective lens for projecting a light beam whose cross-sectional beam diameter is set to a predetermined size onto the recording surface of an optical disk as a recording medium. The optical disk apparatus irradiates the light beam on the recording surface, and extracts reflected light corresponding to information recorded on the optical disk, thereby reproducing the information.
The above optical head apparatus is comprised of a semiconductor laser element (to be simply referred to as a laser element hereinafter) serving as a light source for emitting a light beam, an objective lens for focusing the light beam emitted from the laser element onto the recording surface of an optical disk as an information recording medium and extracting reflected light beam reflected by the recording surface, a photodetector for photoelectrically converting the reflected light beam extracted by the objective lens and outputting a reproduction signal corresponding to the information recorded on the optical disk, a plurality of optical members forming the optical paths of light beams, and the like.
Guide grooves called grooves are formed in the recording surface of an optical disk to make the focused beam spot of the light beam focused by the objective lens always follow a predetermined position in the radial direction.
Known tracking control is performed to move the objective lens in the radial direction of the optical disk so as to make the center of the focused beam spot focused by the objective lens always coincide with the center of such a groove.
In this case, the amount by which the objective lens is to be moved, i.e., the tracking control amount, is set on the basis of the tracking error signal obtained by using, for example, the known push-pull method. Note that the push-pull method is disclosed in, for example, FIG. 1.99 in Noboru Murayama, "Optical Disk Technology", Radio Technology, 1989, pp. 86-88.
Header fields in which ID portions for providing address information, SYNC portions for providing sync signals, and the like are formed as pre-pits on an optical disk, as disclosed in, for example, Masahiro Ojima et al., "Principles and Applications of Optical Storage", 1995 IEICE, pp. 140 and 141. These header fields are located at predetermined positions in grooves at predetermined intervals. Note that the header fields are generally formed as pit rows having various predetermined lengths without offset in a direction in which the grooves extend when viewed from the circumferential direction.
As demands have recently arisen for optical disks having higher recording densities, a method (land/groove recording method) of recording information between grooves, i.e., on lands, as well as in grooves has been proposed. This method is disclosed in more detail in, for example, Sadatoshi Hujiwara et al., "Next-Generation Optical Disk Technology", Trikeps, 1995, pp. 59-74.
As a header field arrangement method in the land/groove recording method, a method of forming a total of four header fields shifted to the inner and outer peripheral sides, in pairs, by 1/4 the groove pitch has been proposed in, for example, PROCEEDINGS OF THE 1995 IEICE GENERAL CONFERENCE, Section C-287. According to this proposal, the track position and the like are detected from all the information recorded on the four header fields.
In the above land/groove recording method, data are recorded (recorded data are present) on lands and grooves. In other words, a focused beam spot must trace lands and grooves.
In this case, whether a focused beam spot is tracing a land or a groove must be discriminated on the basis of the information recorded on the header fields. As is known, the output from a photodetector having division lines formed along the circumferential direction of the optical disk is extracted as a difference signal based on the received light pattern, thereby detecting the position of the focused beam spot. More specifically, every time the focused beam spot passes through a pit of a header field, a reproduction signal waveform having a given amplitude is output, including, for example, a positive displacement component when the header field is shifted to the inner peripheral side of the groove, or a negative displacement component when the header field is shifted to the outer peripheral side of the groove. By specifying the polarity of this displacement component, therefore, a land or a groove is identified.
If it is necessary to read information from a position (groove or land) different from the current track (groove or land) on the optical disk or record new information in another track, the focused beam spot must be moved from the track on which the focused beam spot is currently located to a predetermined target track.
In many cases, this movement is controlled by appropriately combining two operations, i.e., driving an actuator in the radial direction by using a linear motor and displacing the objective lens on the actuator in the tracking direction (to be referred to as a lens shift hereinafter).
To realize high-speed information read or write, it is desirable to make a focused beam spot follow the track center as quickly as possible when the focused beam spot reaches a desired track. To realize this, the following method is generally used. First of all, the overall actuator is coarsely brought close to the target track, e.g., 5 to 10 tracks before the target track, by the linear motor. For movement corresponding to the several remaining tracks, tracking is controlled by a lens shift. As the focused beam spot approaches the target track, or after the focused beam spot reaches it, the linear motor is finely driven to reduce the lens shift amount.
In this method, however, when the above lens shift is performed to make the objective lens trace the target track, an offset component is superimposed on a phase difference signal. As a result, a track offset occurs; the track center is determined even if the center of the focused beam spot deviates from the center of the target track. This makes tracking control unstable.
In addition, in detecting a track deviation signal by the push-pull method, in spite of the fact that the track center coincides with the center of the focused beam spot, an unwanted track deviation signal (false track deviation signal) indicating the occurrence of a track deviation is output. This also makes tracking control unstable. If the false track deviation signal and the track offset have the same polarity, a tracking error may be caused by tracking control.
In contrast to this, in the land/groove recording method, since the signal from each header field includes a component that can be identified as either a displacement component, i.e., the above track offset, or the false track deviation signal, the true displacement component must be accurately separated. That is, in land/groove recording, if the true displacement component is not separated, a reproduction signal from a given header field on the inner or outer peripheral side is buried in the above track offset or false track deviation signal. This causes a decrease in reproduction precision of the information recorded on the header field, or a read error or failure in the worst case.